1. Field of the Invention
The present invention relates to a collimator lens used in a light-scanning apparatus for scanning a laser beam so as to record or display images, such as copier, laser printer, or the like; and, more specifically, to a collimator lens for converting a divergent luminous flux emitted from a light source such as semiconductor laser into a parallel luminous flux, and a light-scanning apparatus using the same.
2. Description of the Prior Art
There have conventionally been known various kinds of light-scanning apparatus for scanning a laser beam so as to record or display images, such as copier, laser printer, and the like.
Such a light-scanning apparatus is configured such that the laser beam emitted from a semiconductor laser is converted into a parallel luminous flux by a collimator lens, the resulting luminous flux is deflected in response to the rotation of a rotary polygon mirror, and thus deflected luminous flux is focused by an f.theta. lens onto an imaging surface.
Here, as disclosed in Japanese Unexamined Patent Publication Nos. 58-14109, 58-38915, 61-279820, 61-273520, and 2-73324, for example, a two-group, two-element collimator lens has been known, which aims at reducing the weight and dimensions of the lens.
It is necessary for such a collimator lens used in the light-scanning apparatus to be a bright lens system in order to enhance the efficiency of utilization of light from the light source and increase the illuminance on the photosensitive drum surface. As a consequence, the collimator lens generally has a larger aperture size as compared with the f.theta. lens system, thereby being likely to generate greater aberrations. Therefore, it is necessary for the collimator lens to favorably correct these aberrations such as wavefront aberration.
As the light-scanning apparatus, there has been known a multi-beam system in which a plurality of light sources are used for multi-beam scanning, thereby enabling higher scanning speed or simultaneous recording of different kinds of information by a single scanning operation. In such cases where this multi-beam system is used and the like, it is desirable that aberrations be favorably corrected within the range of about 2 degrees of half angle of view .omega..
In the collimator lenses disclosed in the above-mentioned publications, however, examples having a narrow angle of view are prevailing, as evidenced by the fact that their performances with respect to off-axis light are only taken into consideration for an angle of view corresponding to the installation error, for instance. Their examples having a wide angle of view yield greater aberrations, thus being hard to be applied to a multi-beam scanning optical system in which a plurality of light sources are disposed on a plane perpendicular to the optical axis, for instance.
Further, in the collimator lenses disclosed in the above-mentioned publications, back focus, i.e., the distance from the light-source-side lens to the light source, is short, i.e., approximately 0.4 f to 0.6 f.
When back focus is short, the collimator lens is located near the light source (semiconductor laser or the like), whereby the temperature of the collimator lens is likely to rise due to the heat from the light source. As a consequence, there has been a demand for increasing the distance from the collimator lens to the light source so that the former is less likely to be influenced by the heat from the latter.